Better Assassin Armour Style?

I cannot decide whether I should buy the Valkyrie or the Nightstalker style of Assassin armour. Which one is best to buy?

What are the advantages and disadvantages to each.

ps. I am usually an assassin that's near the front lines.

IMO the nightstalkers is better if, as you said, you're "near the front lines." (Which, honestly, an assassin should never be but hey, you're the one playing.) The extra armor while attacking can make a nice difference.

I go with the +armor while attacking for the chest and legs and use the +energy for the arms and feet.